Fixed #2 -- Make sure setup.py will work before dateutil is installed.

Thanks to stefanhenze for the report.
Fixed #3 - Cleaned up exception handling to match Xero docs.

Tests have been added for all exception types; where possible, based on actual responses from
the Xero servers.

Thanks to stefanhenze for the report.
Fixed #4 -- Added documentation on how to run the test suite.

Thanks to shamrin for the report.
Fixed #5 -- Corrected handling for non-ASCII data in XML responses.

Thanks to shamrin for the report and the draft patch.